The invention relates to a process and system to reduce emissions of nitrogen oxides and mercury and to reduce the level of carbon in combustion fly ash. More specifically, the present invention provides a process and system to increase use of fly ash and to decrease nitrogen oxides and mercury from flue gases from combustion systems such as boilers, furnaces and incinerators.
Production of air pollution by combustion systems is a major problem of modern industrial society. The pollution can include particulates such as fine fly ash particles from solid fuel combustion (for example, pulverized coal firing), and gas-phase species, such as oxides of sulfur (SOx, principally SO2 and SO3), carbon monoxide, volatile hydrocarbons, nitrogen oxides (mainly NO and NO2 collectively referred to xe2x80x9cNOxxe2x80x9d) and volatile metals such as mercury (Hg).
The nitrogen oxides are the subject of growing concern because of their toxicity and their role as precursors in acid rain and photochemical smog processes. NOx is emitted by a variety of sources, including mobile sources (such as automobiles, trucks and other mobile systems powered by internal combustion engines), stationary internal combustion engines and other combustion sources such as power plant boilers, industrial process furnaces and waste incinerators. Available NOx control technologies include Selective Catalytic Reduction (SCR) and Combustion Modification. SCR systems can be designed for most boilers and may be the only approach for high NOx units such as cyclones. Combustion Modification achieves deep NOx control by integrating several components. Typically, Low NOx Burn (LNB) is the lowest cost Combustion Modification technique. It is usually applied as a step towards low cost deep NOx control. Other Combustion Modification techniques include Overfire Air (OFA), Reburning and Advanced Reburning.
Mercury is identified as a hazardous air pollutant and is the most toxic volatile metal in the atmosphere. Elemental mercury vapor can be widely dispersed from emission sources. Other forms of mercury pollutants include organic and inorganic compounds that accumulate in plants and animals. Mercury is a constituent part of coal mineral matter. Its emission from coal-fired power plants is suspected to be a major source of environmental mercury.
Thus, there is a need to continue to use low NOx technologies but to effectively control mercury emission.
The invention provides an integrated method and system for reducing NOx environment emissions and mercury environment emissions. In the method, a factor is selected to control a combustion process to generate a flue gas comprising fly ash with enhanced unburned carbon; the combustion process is controlled according to a factor selected from reburning fuel, flue gas temperature, OFA injection, coal particle size, LNB flow, LNB design, combustion zone air, stoichiometric ratio of fuel, fuel/air mixing in a primary combustion zone and fuel/air mixing in a secondary combustion zone to produce the flue gas comprising fly ash with enhanced unburned carbon and to vaporize mercury; and the flue gas is allowed to cool to collect fly ash with enhanced unburned carbon with absorbed mercury.
In an embodiment, the method decreases emissions of nitrogen oxide and mercury while decreasing carbon in fly ash. The method comprises selecting a combination of factors from the group consisting of fuel type, fuel/air staging and a combustion condition to control a combustion process to generate a flue gas comprising fly ash with enhanced unburned carbon; controlling the combustion process according to the factors to produce the flue gas comprising fly ash with enhanced unburned carbon, NOx and vaporized mercury; removing NOx from the flue gas; allowing the flue gas to cool to a lower temperature to collect fly ash with absorbed mercury; passing the fly ash with absorbed mercury through an ash burnout unit to remove carbon from the fly ash and to produce a mercury-containing exhaust gas; and passing the mercury-containing exhaust gas through a collection unit to capture the mercury.
Additionally, the invention relates to a system to decrease emission of mercury; comprising a combustion zone that is controlled to generate a flue gas comprising fly ash with enhanced unburned carbon and that produces vaporized mercury; and a post combustion zone to cool the flue gas to collect fly ash with enhanced unburned carbon with absorbed mercury.
In another embodiment, the invention is a system to decrease emissions of nitrogen oxide and mercury while decreasing carbon in fly ash, comprising a combustion zone that is controlled by fuel type, fuel/air staging or a combustion condition to generate a flue gas comprising fly ash with enhanced unburned carbon and that produces vaporized mercury; a post combustion zone to cool the flue gas to collect fly ash with enhanced unburned carbon with absorbed mercury; an ash treatment unit that removes carbon from the fly ash and produces a mercury-containing exhaust gas; and a collection unit that captures the mercury.